1. Field of the Invention
This invention relates to coating compositions for electrophotographic carriers which can impart good durability to carriers on repeated use in duplication. The invention also relates to carriers for electrophotography which have core particles and a cover film formed on each particle from the coating composition.
2. Description of the Prior Art
As is well known in the art, the developers of two-component dry duplicators are usually made of two components including a fine toner and a larger-size carrier. On frictional mixing of these components under agitation, the toner and the carrier are electrostatically charged negatively and positively or vice versa, respectively. The thus charged toner is electrostatically deposited on the electrostatic latent image formed on a photosensitive material, thereby forming a visible image. The visible image is then transferred to a transfer sheet and fixed thereon, thereby achieving duplication.
In the case, the carrier used may be ordinarily made of oxidized or non-oxidized iron powder. If such an iron powder is used, as it is, by mixing with a toner, there arises the problem that the triboelectric charge characteristic against toner is not satisfactory and that the toner is fixedly deposited on the surfaces of the carrier during use, thereby forming a toner film (i.e., a spent phenomenon), so that the charge characteristic of the carrier varies as time passes, thus the life of the developer being shortened. Another problem is that a great difference in the charge characteristic appears between the carriers under dry and wet conditions.
To overcome the above problems, the current tendency is to cover the carrier particles with resins such as fluorine resins, acrylic resins, styrene-acrylic copolymers, silicone resins, polyester resins and the like.
However, the use of hydroxyl group-containing polyesters or acrylic resins is disadvantageous in that crosslinkage through isocyanates or melamines is essential as is set out, for example, in Japanese Laid-open Patent Application Nos. 59-53875 and 60-59369, and if the crosslinkage does not proceed to a full extent, unreacted hydroxyl groups and isocyanate groups remain in the cured product, degrading the charge characteristic.
Where acrylic resins or styrene-acrylic copolymers are used for the coverage, initial charge characteristics are good but the cover film made of these resins is relatively brittle, resulting in poor durability.
On the other hand, attention has now been directed to fluorine resins or silicone resins in that since their surface energy is small, the spent phenomenon may be lowered such as proposed in Japanese Laid-open Patent Application Nos. 54-21730 and 58-40557 and Patent Publication Nos. 59-26945 and 59-131944.
Especially, with the silicone resins, it is possible to design a diversity of molecular structures by proper selection of monomers. This leads to the fabrication of carriers which have a variety of charge levels depending on the selection of monomer. In addition, the silicone resins can be dissolved in diverse solvents, ensuring uniform application on the surfaces of carrier particles. Depending on the type of silicone resin, it will be possible to cure at relatively low temperatures. Thus, silicone resins have a number of advantages in view of handling and working properties.
However, known methyl-bearing silicone resins are disadvantageous in that when formed as a covering film, the resin is not high in mechanical strength although not so low as that of acrylic resins. During long-term use, the film will come off owing to the frictional abrasion, separation and cracking and thus loses its characteristic properties, with the lowering of copying performance. In this sense, the known silicone resins are not satisfactory with respect to the life in long-term use.
Several types of long-life coating compositions have been proposed such as, for example, in Japanese Laid-open Patent Application Nos. 2-73372 and 2-103563 but are not satisfactory in performance.